Planar interdigitated dielectric sensor

ABSTRACT

A planar interdigitated dielectric sensor useful for measuring the surface properties of a material is disclosed. The sensor is formed on an insulating substrate. Attached to the surface of the substrate is an excitation and response electrode disposed in an interdigitated pectinate configuration. Filling the space between the electrodes is an insulating material of known dielectric properties. The upper surface of the electrodes are generally coplanar with respect to the insulating material between the electrodes thus forming a flat upper surface on the sensor. This flat upper surface serves to eliminate air gaps between the sensor surface and sample when analyzing relatively viscous materials.

This application is a continuation-in-part of application Ser. No.07/274,461, filed Nov. 21, 1988, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a sensor which can be used to measuredielectric properties of materials.

It is well known that by measuring dielectric properties of a sample asa function of temperature, valuable information can be gained concerningthe physical and chemical properties of the sample.

Two techniques are commonly used to measure dielectric properties. Formany years measurements have been made by placing a sample betweenparallel plate electrodes, applying an electrical signal in the form ofan alternating voltage, to one of the electrodes (i.e. the excitationelectrode) and measuring the electrical signal from the other electrode(i.e. the response electrode). The following equation is used: ##EQU1##C=capacitance e_(o) =permittivity of free space (a constant)

e'=permittivity of sample (being measured)

A=area of parallel plate response electrode

d=distance between the excitation and response electrode plates

By measuring capacitance, the permittivity of the sample can be easilycalculated if the area of the parallel plate electrode and the distancebetween the excitation and response electrodes are known. A device formaking measurements in this manner is disclosed in copending applicationSer. No. 07/206,092. This technique is primarily used to characterizebulk properties of a material in that the signal is monitored throughthe entire thickness of the material. This technique has severallimitations. Often times thick samples are of interest to be analyzed.In the parallel plate technique, the signal to noise ratio decreases asa function of increasing distance between the electrode plates. Largerplates could be utilized to increase the area thereby increasing thesignal. However there does exist a practical limitation. Many times thesurface of a material is to be analyzed. In polymer molding, skineffects are of interest due to faster cooling of the material's surfacethan its interior. The chemical and mechanical properties of the surfaceof the material are more indicative of its end use properties than thebulk interior properties. Coatings on a material surface are also ofinterest in dielectric analysis. Paints, adhesives, and copolymers oftenrequire analysis. A parallel plate measurement would detect theproperties of the coating and its associated substrate in a bulkfashion. It is impossible to analyze surface characteristics by parallelplate analysis.

An alternate technique was developed and is commonly known whichaddresses the limitations of the parallel plate measurement. Aninterdigitated combed electrode is commonly used for obtainingdielectric measurements on surfaces of materials and fluids. Probes ofthis type have been used for many years as moisture detection devices.U.S. Pat. No. 3,696,360 to Gajewski, discloses an interdigitateselectrode for moisture sensing. In the past few years theseinterdigitated probe structures were adapted to measure dielectricproperites of materials. See, Society for the Advancement of Materialand Process Engineering Journal, Volume 19, No. 4, July/August, 1983.

In this technique a sample is placed on the electrode surface, anelectrical signal is applied to one "finger" of the interdigitatedfingers or combs of the electrode array, and the signal is measured atthe other finger of the array. These two fingers are termed excitationand response electrodes respectively. In this fashion the signal onlypenetrates the surface of the material. The penetration depth isapproximately equal to the distance separating the fingers in theinterdigitated electrode array. This technique is ideal for monitoringthe dielectric characteristics of fluids, curing systems, adhesives, andrelatively low viscosity materials. This technique, however, has severelimitations when analyzing films, hard plastics, and pre-cured systems(i.e., relatively viscous materials).

Problems arise due to surface contact with these types of materials.Interdigitated surface electrodes are inherently quite sensitive towhatever material contacts the surface, including air. Air gaps canseverely limit the ability to measure dielectric properties of amaterial effectively since air and vacuum have the lowest permittivitytheoretically possible (e'_(vac) =1.00000). Thus, air gaps on theelectrode surface will significantly, depress the measurement of amaterial's permittivity. Air also induces noise in the capacitancemeasurement. Unfortunately all electrode materials used in thefabrication of these surface sensors have a finite height or thickness.Obviously, a hard material will bridge the electrode fingers of thesensor. This bridge traps air between the electrode surface and thematerial in the spaces between the electrode fingers. This causes themeasured permittivity to be the average value of the material'spermittivity and the permittivity of air. This effect can depress thecorrect measurement of permittivity by as much as 50%.

Primarily dielectric measurements are made as a function of temperatureto assess the characteristics of a material. As these viscous materialsare heated beyond the glass-transition region they begin to flow anddisplace the air between the electrode fingers. This results in adramatic elevation in the permittivity measurement. It would seem aviable option prior to analysis to preheat materials so that they "flowout" to fill the air gaps on the electrode to address this problem.Unfortunately, most materials change their molecular structure uponheating and cooling. This change effects the accurate measurement ofglass transition temperature, degree of crystallinity, degree of cure,and most importantly, permittivity. Unfortunately the amount of airpresent between the electrode surface and the material being analyzedchanges as the material softens with temperature. If the amount of airin this space remained constant a calibration could be made and themeasurement could be corrected.

A surface analysis dielectric sensor is needed to eliminate the abovementioned limitations of existing interdigitated pectinate probes foruse with relatively viscous materials.

SUMMARY OF THE INVENTION

Provided by this invention is an improved surface analysis dielectricsensor with the following properties:

(a) an interdigitated, combed electroded configuration supported on aninsulating substrate;

(b) a surface being generally planar and flat to within microns, therebyeliminating air gaps between the surface of the electrode and anymaterial of interest;

(c) an embodiment which allows accurate dielectric measurements over anextreme temperature range.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic describing a parallel plate sensor configurationwith sample.

FIG. 2 is top view of prior art.

FIG. 3 is a side elevational view of surface dielectric sensors of theprior art.

FIG. 4 is a side elevational view which shows a sample bridging thesurface sensor of the prior art.

FIG. 5 is a side elevational view of the present invention.

FIG. 6 is a side elevational view of a sample under analysis with thepresent invention.

FIG. 7 is a side elevational view of present invention manufactured byan alternate method.

DETAILED DESCRIPTION

Referring now to the drawing in FIG. 1, the schematic shows a typicalparallel plate arrangement wherein the sample S is sandwiched between anexcitation electrode plate 10 and a response electrode plate 12. Asdiscussed in the background, only the bulk characteristics of a materialcan be analyzed in this technique. FIG. 2 shows a surface analysissensor of the prior art. The interdigitated array 14 is made up of twocombed shaped electrodes disposed in a pectinate configuration. One suchelectrode being the excitation electrode 18, and the other electrodebeing the response electrode 19. These electrodes are connected toelectrode contacts 17, and are supported on an insulating substrate 16.FIG. 3 is a cross-section of the sensor described in FIG. 2. As can bereadily seen the electrodes 18 and 19 extend a distance D above thesurface 20 of the substrate 16. FIG. 4 shows a relatively viscous sampleS' under analysis with the sensor of FIGS. 2 and 3. As is shown,unavoidable air gaps 21 are created as the sample S' bridges theelectrode fingers 18 and 19 since the sample S' does not readily flow.FIG. 5 shows a sensor of the present invention. The surface 22 of thesensor is generally planar and flat. The void between the excitationelectrode 18' and the response electrode 19, contains a dielectricmaterial 24 with known dielectric properties. FIG. 6 shows a sample S''under analysis with the sensor of FIG. 5. As is shown, no air gaps existbetween the sensor surface and the sample being analyzed.

The substrate 16' of our invention is any of a number of insulatingmaterials, well known to those skilled in the art such as polyimidefilm, ceramic, or glass. The electrodes 18' and 19' of our invention aremetallic AC voltage accepting electrodes. As described above theseelectrodes are these electrodes are combed-shaped and disposed in ainterdigitated pectinate configuration. These electrodes are attached tothe upper surface of the insulating substrate 16' The electrodes 18' and19' have a finite height and a flat upper surface. These metallicelectrodes are conductors such as gold, platinum, copper, palladium, andso forth.

The dielectric material 24 with known dielectric properties between theelectrodes is an insulating glass such as those commonly used in themanufacture of thick film hybrid circuits. One such insulating glass ismanufactured by E. I. du Pont de Nemours and Company as thick filmdielectric composition No. 5034 or No. 5042.

There are at least two basic manufacturing methods which can be used tomanufacture the sensor of the present invention. Thick film hybridtechnology using screen printing processes is known in the art and canbe used to form the electrode array, as well as to selectively fill thespaces between the electrodes. A flat generally planar surface isobtained by filling the spaces between the electrodes with excessdielectric insulating material. The sensor is then lapped to the desiredsurface flatness. Another method of manufacturing the sensor of thepresent invention utilizes etching technology. In this method groovesare etched by chemical or mechanical means into the surface of thesubstrate. The grooves are combed-shaped and disposed in interdigitatedpectinate configuration. The grooves are then filled with the metallicconductors. A sensor obtained using this method is shown in FIG. 7. Inthis configuration, the insulating material between the excitationelectrode 18'' and the response electrode 19'' consists of the substrate16'' itself.

Many alternate manufacturing methods would be obvious to one skilled inthe art.

I claim:
 1. A planar interdigitated dielectric sensor useful formeasuring surface properties of a material comprising:(a) an insulatingsubstrate; (b) a metallic AC voltage accepting excitation electrodeattached to the surface of said substrate, the excitation electrodebeing comb-shaped and having a planar upper surface; (c) a metallicresponse electrode attached to the surface of said substrate, theresponse electrode being comb-shaped, having a planar upper surfacecoplanar with said planar upper surface of the excitation electrode andinterdigitatedly positioned with respect to said excitation electrode;and (d) an insulating glass with known dielectric properties attached tothe surface of the substrate, filling the space between the excitationand response electrodes and having an upper surface coplanar withrespect to the coplanar upper surfaces of said excitation electrode andsaid response electrode, whereby the upper surfaces of the excitationelectrode, the response electrode, and the insulating glass are adaptedfor placement against a surface of a viscous material that is beingtested such that no air gaps are present between said upper surfaces andthe surface of the viscous material.
 2. A planar interdigitateddielectric sensor useful for measuring surface properties of materialscomprising:(a) an insulating substrate having a planar upper surface,said upper surface containing a first recessed groove having a comb-likeshape and a second recessed groove also having a comb-like shape, saidgrooves being positioned interdigitatedly; (b) a metallic responseelectrode with a planar upper surface positioned within said firstgroove, said planar upper surface of the response electrode beingcoplanar with respect to the upper surface of said substrate; and (c) ametallic excitation electrode with a planar upper surface positionedupper surface of the excitation electrode being coplanar with respect tothe upper surface of said substrate, whereby the upper surfaces of theexcitation electrode, the response electrode, and the substrate areadapted for placement against a surface of a viscous material that isbeing tested such that no air gaps are present between said uppersurfaces and the surface of the viscous material.
 3. The planarinterdigitated dielectric sensor of claim 1, wherein the surfaces of theexcitation electrode, the response electrode, and the insulating glassare coplanar and flat within microns.
 4. The planar interdigitateddielectric sensor of claim 1, wherein the electrodes are chosen from agroup consisting of gold, platinum, copper and palladium.
 5. The planarinterdigitated dielectric sensor of claim 2, wherein the surfaces of theexcitation electrode, the response electrode, and the substrate arecoplanar and flat within microns.
 6. The planar interdigitateddielectric sensor of claim 2, wherein the substrate is a dielectricmaterial with known dielectric properties.
 7. The planar interdigitateddielectric sensor of claim 2, wherein the electrodes are chosen from agroup consisting of gold, platinum, copper, and palladium.
 8. A methodfor measuring the surface dielectric properties of viscous materialscomprising the steps of:(a) applying a planar dielectric sensorcomprising (i) an insulating substrate, (ii) a comb-shaped metallic ACvoltage accepting excitation electrode having a planar upper surfaceattached to an upper surface of said substrate, (iii) a comb-shapedmetallic response electrode having a planar upper surface attached tothe upper surface of said substrate and interdigitatedly positioned withrespect to said excitation electrode, and (iv) a dielectric materialwith known dielectric properties attached to the upper surface of thesubstrate, filling the space between the excitation and responseelectrodes and having an upper surface coplanar with respect to theplanar upper surface of said excitation electrode and the planar uppersurface of said response electrode, against a surface of the material tobe tested such that no air gaps are present; (b) applying an alternatingcurrent electrical signal to the excitation electrode; (c) detecting aresponse signal at the response electrode; and (d) monitoring thedielectric characteristics of the viscous material.
 9. The method formeasuring the surface dielectric properties of viscous materials ofclaim 9, further comprising the step of monitoring the dielectriccharacteristics of the viscous material as a function of temperature.10. A method for measuring the surface dielectric properties of viscousmaterials comprising the steps of:(a) applying a planar dielectricsensor comprising (i) an insulating substrate having a flat uppersurface, said upper surface containing a first recessed groove having acomb-like shape and a second recessed groove also having a comb-likeshape, said grooves being positioned interdigitatedly, (ii) a metallicresponse electrode with a planar upper surface positioned within saidfirst groove, said planar upper surface of the response electrode beingcoplanar with respect to the upper surface of said substrate, and (iii)a metallic excitation electrode with a planar upper surface positionedwithin said second groove, said planar upper surface of the excitationelectrode being coplanar with respect to the upper surface of saidsubstrate and with respect to the upper surface of said responseelectrode, against a surface of the material to be tested such that noair gaps are present; (b) applying an alternating current electricalsignal to the excitation electrode; (c) detecting a response signal atthe response electrode; and (d) monitoring the dielectriccharacteristics of the viscous material.
 11. The method for measuringthe surface dielectric properties of viscous materials of claim 10,further comprising the step of monitoring the dielectric characteristicsof the viscous material as a function of temperature.